Arrangement for minimizing negative signals



Aug. 21, 1956 J- T. DALTON ET AL ARRANGEMENT FOR MINIMIZING NEGATIVESIGNALS Filed April 20, V 1955 2 Sheets-Sheet 1 INVENTOR.

JAMES T. DALTON BY AND RICHARD H. STEVENS 4 M ATTORNE Y Aug. 21, 1956 J.T. DALTON EI'AL 2,760,076

ARRANGEMENT FOR MINIMIZING NEGATIVE SIGNALS 2 Sheets-Sheet 2 Filed April20, 1955 INVENTOR.

JAMES T. DALTON ATTORNEY GEMENT FOR MINIMIZING NEGATIVE SIGNALSApplication April 20, 1955, Serial No. 502,756 8 Claims. (Cl. 250-413)This invention relates to mass spectrometers and more particularly to acollector arrangement for mass spectrometers which overcomes theinaccuracies in analysis and/or calibration resulting from strayelectrons, negative ions or other charged particles in the generalregion of the collector assembly.

In the operation of mass spectrometers, negative signals, commonlycalled negative peaks or base line depressions, often appear in theoutput signal from the collector. On a mass spectrum scan record, theseappear as traces extending below the base line, that is, the line tracedwhen the ion output from the source is zero. These signals are believedto be the result of secondary emission of electrons from metal surfacesbombarded by the ion beam and, to a degree, the production of negativeions. Signals of this character are objectionable, particularly in thecase of highly-sensitive spectrometers, because they tend to make thepositive peak height in the collector output indefinite and obscure thebase line level. Consequently, negative peaks make calibration difiicultand reduce accuracy.

Heretofore in the prior art, the problem of or the necessity for theelimination or disposition of undesired ions and other charged particleshas been recognized. Nier, 2,660,677; Long, 2,543,859; and Washburn,2,598,734. Magnets have been suggested for use at the collector end ofthe spectrometer tube to dispose of stray or undesired charged particlesby positioning such magnets so that the magnetic field is generallyparallel to the slit at the collector slit plate, thereby causing thepaths of the undesired particles to bend or to be deflected. Nier,supra. The object is to have the field act on the charged particles tocause them to travel in paths which will bring them into contact withgrounded conductive surfaces that will serve to neutralize and disposeof them It has also been attempted to dispose of undesired chargedparticles at the collector end of the spectrometer through the use ofcharged suppressor or suppressor and grounded electrodes positionedeither adjacent to or on either side of the collector plate. Long,supra; Washburn, supra. Further, it is known that during the operationof narrow range or multiple mass spectrum range mass spectrometersnegative signals that appear in the mass range being used can be reducedby manipulation of the focusing potentials on the ion source of theinstrument. By refocusing, these negative signals can be made to shiftand appear in other mass ranges not being used at the particular time ofoperation. While these arrangements have had some eflect in reducingspurious or interfering peaks which tend to mask the true signal andhave resulted in reducing the shift in the base line, such arrangementsand practices have proved to be inadequate for overcoming the effects ofnegative peaks and base line depression for spectrometers which are bothhighly sensitive and adapted for the continuous uninterruptedmeasurement of a wide range of masses e. g. from mass 12 to mass 600.

Applicants with a knowledge of these problems in the nitcd States Patent2,760,075 Patented Aug. 21, 1956 "ice prior art have for an object oftheir invention the provision of an improved arrangement for eliminatingnegative peaks in the output signal from these mass spectrometers.

Applicants have as another object of their invention the provision of atrap and magnet assembly for a mass spectrometer to capture and disposeof undesired charged particles as they approach the collector.

Applicants have as a further object of their invention the provision ofa collector assembly and magnet for disposing of stray or secondaryelectrons and the elimination of negative signals by altering the pathsof travel of undesired charged particles and capturing them whilepermitting the desired ion beam to travel therethrough.

Applicants have as a still further object of their invention theprovision of a collector arrangement for a mass spectrometer employing aplurality of electrodes associated to define adjacent electron capturechambers and an external magnet for disposing of electrons whilepermitting the passage of the ion beam therethrough.

Applicants have as a still further object of their invention theprovision of an electrode and magnet assembly for disposing ofundesirable charged particles in a mass spectrometer at the collector,employing a series of spaced plate electrodes having aligned slits ofprogressively narrower Width and an external magnet.

Other objects and advantages of our invention will appear in thefollowing specification and accompanying drawings, and the novelfeatures thereof will be particularly pointed out in the annexed claims.

In the drawings, Figure l is a sectional elevation of a conventionalmass spectrometer incorporating our improved electron trap and magnetassembly. Figure 2 is a cross-sectional view taken along the line 2-2 ofFigure 1. Figure 3 is a comparative chart indicating generally theefiect of Wave shapes at the collector with and Without applicantsimproved assembly. Figure 4 is a schematic of our improved collectorassembly indicating its action on undesired charged particles. Figure 5is an exploded view of our improved collector assembly for a massspectrometer. Figure 6 is a schematic of our improved collector assemblyshown associated with a collector electrode.

While applicants invention may have application to many special types ofspectrometers, including process monitoring recorders, it also hasgeneral application to analyzers of the spectrometer type, and in Figurel a tube of a mass spectrometer of a conventional type is designated 10and includes a glass envelope sealed to the copper analyzer tube 11. Thetube portion 12 housing the collector assembly 22 and collector plate 21may be made of any appropriate material, while the collector assembly 22and collector plate 21 is preferably made of Nichrome V and can beappropriately joined to the copper tubing portion of the spectrometer.The gas to be analyzed is fed into the instrument through the gas inletor sample tube 13, and vacuum is maintained in the spectrometer tube bypump down through outlet tube 14 which communicates with the usualarrangement of vacuum pumps (not shown). The sample tube 13 leads to thesource 18 and enters the ionization chamber 17 thereof. This chamber isbetter illustrated in Figure 2 where the filament F is heated to give anelectron beam which traverses chamber 17 and is received on anode orplate T. The electron beam passes over and is located immediately abovethe slit 27 in the shield plate S.

Ions formed by collision of the electrons with the gas molecules aredrawn through the slit 27 in plate S and accelerated through a series ofplates J1, J2, J3, J5, and G. Plates J1 and I2 are a split pair andpermit one to bend the beam to one side or the other to compensate forimperfections in the alignment as well as for the slight bending of thebeam produced by the magnetic field used for aligning the electron beam.The remainder of the plates form a lens which not only increases theintensity of the ion beam entering the magnetic analyzer, but alsoprevents a dropping off of ion intensity for all but the lowest energyions. The upper plate marked G is grounded along with the magneticanalyzer housing to which it is tied.

The magnetic field associated with the tube for bending the ion beam,and that resulting in dispersion of the ions of various masses, is intrapezoidal form as indicated at 20, the pole faces being indicated bynumeral 19 on Figure 2. The collector plate is indicated at 21 and thisis associated with our improved collector assembly 22 which ispositioned in front of the collector plate. The lead 23 from thecollector plate extends through the glass insulator 26 and passesoutwardly from the tube through the shielding element 24. The magneticfield is set up by any conventional type of magnet. The magnet 9,however, is preferably a U-type permanent magnet assembly whole polesare positioned against the sides of the spectrometer tube and consistsof two rod magnets in axial alignment connected by a yoke of soft iron,with field strength of approximately 200 to 350 gauss. The field isgenerally parallel to the major axis of the collector slit. In someinstances, however, best operation is achieved when the field forms anangle of as much as 90 with the major axis of the slit and the exactsetting is rather critical at the particular zone of location. To enablethe magnet to set up the field in this manner, the poles of the magnetare made adjustable for movement in an arcuate path about the tube 12,as well as longitu? dinally thereof.

In its operation, gas molecules which are fed to the source 18 throughthe sample tube 13 are ionized in the ionization chamber. The ions areejected from the chamber 17 through the slit 27 and pass downwardlythrough the beam forming and focusing mechanisms J 1, J2, J3, J5, and G.They are then projected into the portion 11 of the tube which istraversed by a magnetic field whose lines of force extend into the planeof the paper. This field is set up by the conventional spectrometermagnet indicated generally at 20. This action has the elfect of causingthe ions to travel in arcuate paths whose radii correspond to therespective masses of the ions. By adjusting the focusing potentials ofthe source 18 any desired ion beam may be focused on the aligned slitsof the collector assembly 22 and the selected beam may be made to fallor impinge upon the collector electrode 21.

Referring to Figure 4, showing a schematic of our improved collectorassembly with paths of travel of the ions generally indicated by thedotted lines, it will be seen that the field of the magnet 9 with polefaces designated M, M will traverse the spectrometer at the collectorend but is not sufliciently strong to have very much eflect on therelatively high energy ions that make up the beam, so that the main bodyof these ions pass on to the collector plate designated 21. However,some of the ions of the beam, designated B, do not follow the othersthrough the aligned slits to the collector assembly, but strike portionsof the mass spectrometer including the baflle or collector arrangement1, 2 and 3 and knock out or otherwise produce secondary ions orelectrons. These ions and electrons are of relatively low energy, sothat they may be effectively acted upon by the magnetic field, set up bythe magnet M, M, which tends to bend or deflect them. These and otherlow energy ions tend to follow paths which bring them into contact withthe collector arrangement so that they may be neutralized and disposedof. This latter process is aided by the fact that the collectorstructure is arranged to define chambers which tend to trap thesecharged particles and increase their chances of engagement with theconductive metal walls.

Referring now to Figure 5, the aforementioned special baflje assemblycomprises a grounded collector slit plate 1,

a baffle plate 2, and a skirted baffle plate 3. As shown, the baflieplates 2 and 3 are formed with central slits generally similar in shapeto the collector slit. The three slits may he of the same length, butthe slit in the baffle plate 2 is made somewhat wider than the slit 1.The slit in the skirted baffle 3 is made still wider. All three of theslits are formed with very thin edges to minimize secondary electionemission by ions grazing the slit edges. As shown, the baffles 2 and 3are mounted to the collector slit plate 1 by means of suitable elongatedspacer studs 4 and screws 5. With this arrangement, the three slits arepositioned one over another, in axial alignment.

As shown, the central part of the upper face of the collector slit plate1 is cut away, or counterbored, so that the collector slit is recessedwith respect to the upper face. Consequently, when the bafile plate 2 ismounted flush upon the plate 1, a chamber is formed between the centralportions of the plates. This chamber, in conjunction with the chamberbetween baffle plates 2 and 3 apparently functions as an electron trap.It will be noted that the skirt of the battle 3 extends to the upperface of the plate 1, so that the slits in the bafiles 2 and 3 constitutethe only openings through which positive ions, negative ions, andsecondary electrons can enter the collector slit.

The combination of a collector magnet 9 and the assembly 22 ofelectrodes of Figures 1 and 5 are highly eifective in minimizingnegative peaks. For example, this combination is used with line recorderspectrometer systems for gaseous diffusion cascades. These systems arehighly sensitive and are designed to record masses ranging from at leastmass 12 to mass 400. When provided with the above arrangement, thesesystem can be calibrated so that an ion signal as great as 20,000millivolts will result in a negative signal extending less than 16millivolts below the base line on the recorder chart (millivolts wherepreamplifier input resistor equals 10 ohms). This magnitude of inputresistor provides a signal sensitivity greater than is generallyobtained in the art. This is a marked improvement over the prior art,and is obtained without significant reduction in the over-allsensitivity of the systems. Thus under comparable conditions the use ofapplicants improved system results in the reduction of the magnitude ofthe negative peaks by a factor of about 1250.

Now referring to the schematic of Figure 6, it will be seen that thecollector arrangement 22 which includes electrodes 3, 2 and 1 are allmaintained at ground potential while collector electrode 21 is coupledinto the control grid of a D. C. amplifier 41, and by virtue of the dropacross grid resistor 40, is maintained at a potential slightly aboveground when a positive ion beam impinges upon it. It will be seen thatsome electrons or other stray ions which enter the chamber defined orenclosed by skirted electrode 3 may be deflected by the magnetic fieldat the collector assembly since it passes across the chamber substantially parallel with the major axis of the slits of the electrodes3, 2 and 1 and will have the tendency to deflect them to the groundedwalls of the electrode 3 where they are neutralized. Most of the balanceof these particles which enter the chamber defined by electrodes 1, 2are likewise deflected by the magnetic field and pass to ground.

Figure 3 is a chart indicating the eflect of negative peaks on the shapeof signal traces resulting from signals appearing at the collector plate21. It will be seen from chart a that negative peaks can serve tointroduce inaccuracies into the measurements by reduction of thepositive peak height. This is especially true where the negative signalsare so large that the trace goes off of the recording tape. The waveshape of chart b indicates how the wave might have appeared except forthe inaccuracies introduced by the presence of the negative peaks.

Experience has shown that: (a) Better elimination is obtained if theslit in plate 2 is wider than the collector slit, and if the slit in theplate 3 is wider than the slit in (b) The edges of the various slits arepreferably approximate knife edges. (0) The spacing between plate 1 andplate 2 is of critical importance. Whereas the spacing between plates 2and 3 is of somewhat lesser importance. (d) Improvement has been notedand negative signals reduced where the surfaces of the collectorarrangement 22 exposed to positive ion bombardment is locatedperpendicular to the beam.

Having thus described our invention, we claim:

1. A mass spectrometer comprising an elongated envelope, an ion sourcepositioned at one extremity of the envelope for producing ions, meansfor accelerating the ions alog the envelope, a magnetic field threadingan intermediate portion of the envelope for separating the ions intobeams, a collector toward the other extremity of the envelope forcollecting ions of a desired beam, a grounded collector assemblyincluding a plurality of spaced plates having aligned aperturespositioned in the ion beam ahead of the collector for the passage of thebeam, and an additional magnetic field threading the assembly normal tothe beam for directing low energy ions and electrons toward the assemblyfor neutralizing them.

2. A mass spectrometer comprising an elongated tube, an ion sourcepositioned at one extremity of the tube for producing ions, means foraccelerating the ions along the tube, a magnetic field threading anintermediate portion of the tube for separating the ions into beams, acollector toward the other extremity of the tube for collecting ions ofa desired beam, a grounded collector assembly including a plurality ofspaced plates having apertures of substantially the same lengthpositioned in the ion beam ahead of the collector for the passage of thebeam, and an additional magnetic field threading the assembly normal tothe beam for directing low energy ions and electrons toward the assemblyfor neutralizing them.

3. A mass spectrometer comprising an elongated envelope, an ion sourcepositioned at one extremity of the envelope for producing ions, meansfor accelerating the ions along the envelope, a magnetic field threadingan intermediate portion of the envelope for separating the ions intobeams, a collector toward the other extremity of the envelope forcollecting ions of a desired beam, a grounded collector assemblyincluding a plurality of spaced plates having progressively widerapertures positioned in the ion beam ahead of the collector for thepassage of the beam, and an additional magnetic field threading theassembly normal to the beam for directing low energy ions and electronstoward the assembly for neutralizing them.

4. A mass spectrometer comprising an elongated envelope, an ion sourcefor ionizing neutral vapors positioned at one extremity of the envelopefor producing ions, accelerating and collimating electrodes foraccelerating the ions along the envelope, a magnetic fiield threading anintermediate portion of the envelope for separating the ions into beams,a collector toward the other extremity of the envelope for collectingions of a desired beam, a grounded collector assembly including aplurality of spaced plates having aligned apertures positioned in theion beam ahead of the collector for the passage of the beam, and amagnet having pole faces positioned on either side of said assembly forsetting up a magnetic field substantially perpendicular to the ion beamfor directing low energy ions and electrons toward the assembly forneutralizn'ng them.

plate 2.

5. A mass spectrometer comprising an elongated envelope, an ion sourcepositioned at one extremity of the envelope for producing ions, meansfor accelerating the ions along the envelope, a magnetic field threadingan intermediate portion of the envelope for separating the ions intobeams, a collector toward the other extremity of the envelope forcollecting ions of a desired beam, a grounded collector assemblyincluding a plurality of spaced slotted plates provided with conductingbridging enclosures positioned in the ion beam ahead of the collector,and an additional magnetic field threading the assembly normal to thebeam for directing low energy ions and electrons toward the assembly forneutralizing them.

6. A mass spectrometer comprising an elongated tube, an ion sourcepositioned at one extremity of the tube for producing ions, means foraccelerating the ions along the tube, a magnetic field threading anintermediate portion of the tube for separating the ions into beams, acollector positioned adjacent the other extremity of the tube forcollecting ions of a desired beam, a grounded collector assemblyincluding a series of plates having aligned apertures for insertion inthe ion beam ahead of the collector, said plates having a conductingenclosure, and an addition- 211 magnetic field threading the assemblynormal to the beam for directing low energy ions and electrons towardthe assembly for neutralizing them.

7. A mass spectrometer comprising an elongated envelope, an ion sourcepositioned at one extremity of the envelope for producing ions, meansfor accelerating the ions along the envelope, a magnetic field threadingan intermediate portion of the envelope for separating the ions intobeams, a collector located adjacent to the other extremity of theenvelope for collecting ions of a selected beam, a grounded collectorassembly including a series of spaced plates having aligned apertures ofsubstantially the same length and diminishing widths in the direction ofthe collector disposed in the beam ahead of the collector, and aconducting shell for enclosing the plates, and an additional magneticfield threading the assembly normal to the beam for directing low energyions and electrons toward the assembly for neutralizing them.

8. A mass spectrometer comprising an elongated envelope, an ion sourcepositoned at one extremity of the envelope for producing ions, means foraccelerating the ions along the envelope, a magnetic field threading anintermediate portion of the envelope for separating the ions into beams,a collector positioned adjacent the opposite extremity of the envelopefor collecting ions of a selected beam, a grounded collector assemblyincluding a plurality of spaced plates having aligned apertures ofsubstantially the same length for insertion in the beam ahead of thecollector, one of said plates being spaced at a substantially greaterdistance than the others, and a conducting skirt on said one plateextending toward the others to enclose the space, and an additionalmagnetic field threading the assembly normal to the beam for directinglow energy ions and electrons toward the assembly for neutralizing them.

References Cited in the file of this patent UNITED STATES PATENTS2,660,677 Nier Nov. 24, 1953

